A conventional transmitter with a Cartesian modulator typically performs the up-conversion on the baseband signals using analog mixers. In an-in-phase (I) channel for such a transmitter, an in-phase digital-to-analog converter (DAC) converts the in-phase digital baseband data signal into an analog in-phase waveform. An in-phase bandpass filter filters the analog in-phase waveform to produce a filtered in-phase data stream that is up-converted by an in-phase mixer. The in-phase mixer mixes the filtered in-phase waveform with a local oscillator (LO) clock signal to produce a radio frequency (RF) in-phase signal. The quadrature-phase (Q) channel for the transmitter is analogous and thus has its own DAC, filter, and mixer. In both channels, the filters and mixers are analog circuits whereas the DACs are partial digital circuits.
This mix of analog and digital circuits has significant consequences in porting a transmitter design from one process node to another as transistors are further miniaturized. In general, a digital circuit design may be ported to new process nodes without considerable re-design whereas an analog circuit typically requires significant re-design to be ported to such new process nodes. Moreover, the transistors in analog circuits are often difficult to scale down without affecting performance, which results in the analog circuits demanding larger die space as compared to digital circuits. In addition, digital circuits typically benefit from scaling and consume less power than comparable analog circuits. To minimize the need for analog circuits, digitally-intensive transmitters have been developed using RF-DACs. An RF-DAC is a DAC that not only converts a baseband signal into an analog signal but also up-converts the baseband signal into an RF analog signal. An RF-DAC thus not only performs the function of the DAC in a conventional transmitter but also performs the function of the mixer. An example, an RF-DAC-based transmitter 100 is shown in FIG. 1. An in-phase (I) RF-DAC 105 converts an in-phase baseband data stream into an analog in-phase RF signal. Similarly, a quadrature (Q) RF-DAC converts a quadrature baseband data stream into an analog quadrature RF signal. Optional driver amplifiers 115 may amplify the analog I and Q RF signals prior to their combination into an RF output signal amplified by a power amplifier 120. Although such a direct-digital transmitter architecture advantageously eliminates the conventional need for analog mixers and filters, the resulting RF output signal from transmitter 100 will generally suffer from insufficient image suppression and excessive distortion.
Accordingly, there is a need in the art for improved RF-DAC transmitters with improved image rejection and reduced distortion.